Showing papers in "Chemical Engineering in 2009"

Understanding Bends In Pneumatic Conveying Systems

Abstract: solids has been successfully practiced — in industries as diverse as chemical, agricultural, pharmaceutical, plastics, food, mineral processing, cement and power generation — for more than a century. Pneumatic conveying provides advantages over mechanical conveying systems in many applications, including those that require complex routing, multiple source-destination combinations and product containment. Pneumatic conveying transfer lines are often routed over pipe racks and around large process equipment, giving process operators great layout flexibility. Such design flexibility is made possible by the use of bends (such as elbows and sweeps, discussed below) between straight sections (both horizontal or vertical), which enable convenient change of direction in the flow of the conveyed solids. However, among all the components of a pneumatic conveying system, bends — despite their apparent simplicity — are probably the least understood and most potentially problematic for process operators. Findings from various research studies are often not consistent, and often times public findings do not match field experience. The importance of bends in any pneumatic conveying assembly cannot be overstated since — if not properly selected and designed — they can contribute significantly to overall pressure drop, product attrition (degradation) and system maintenance (due to erosive wear). Historically, a basic long-radius bend (shown in Figures 1 and 2, and discussed below) has been the bend of choice for designers of pneumatic conveying systems, for a variety of reasons: • Long-radius bends provide the most gradual change in direction for solids, and hence are most similar to a straight section of piping • The angle of impact on the pipe wall is relatively small, which helps to minimize the risk of attrition or erosion • For lack of other experience, to maintain the status quo Years of field experience and a variety of studies conducted to troubleshoot common problems — such as line plugging, excessive product attrition (degradation), unacceptably high bend wear and higherthan-expected pressure drop — clearly indicate that the flow through bends in pneumatic piping is very complex. One should refrain from generalizing the findings until the underlying physics are well understood. This complexity is exacerbated when innovative designs are introduced to address existing issues with common-radius bends (also discussed below). Today, most of the data still resides with vendors and there is a need for fair, unbiased and technically sound comparative evaluation. The purpose of this article is to summarize the key concepts, outline key metrics used to evaluate bend performance, and provide guidance for their selection. We will limit our discussion to dilute-phase conveying. (Issues related to pipe bends for dense-phase conveying systems will be addressed at a future date.)

Estimating the Total Cost of Cartridge and Bag Filtration

Abstract: ag filters for industrial applications have been in existence longer and are considered by some to be easier and simpler to specify than cartridges for a filtration project. And although cartridge filtration is now one of the mostly widely used filtration technologies in the chemical process industries (CPI), it is not always the first choice. How does one decide which filtration method should be used? Like any other technology choice, this decision is based upon the strengths and weaknesses of the two options. There are many factors an engineer should consider when choosing a filtration system. So when does one specify a cartridge filter instead of a bag filter? What are the basic differences between the two? How does one determine filter life for either type? Often the lack of a logical approach to liquid filtration design leads engineers down a “what did we do the last time” approach instead of determining critical properties, such as the total dirtholding capacity, filter life, filter surface area, flowrates, and other factors. Schooling in this unit operation is not a common university practice, and the lack of ASTM standards, for instance, regarding filtration test procedures and specification of filters adds to system underor over-design. Besides the capital costs of a filter, there are additional factors that affect overall filtration economics, namely: (a) design considerations and options, (b) process requirements, (c) maintenance requirements, (d) maintenance procedures, (e) mean-time-betweenchangeout (MTBC) costs, and (f) disposal costs. This article outlines basic design issues, discusses selection considerations, and presents a cradleto-grave cost analysis of bag and cartridge filtration.

Getting the Most Out of Your Rupture Disc

Abstract: R upture disc devices provide overpressure protection for a variety of storage and process vessels and equipment. The objective of the rupture disc is to maintain a leaktight seal and be a passive bystander until called upon to relieve excess pressure. While this is generally the case, there are times when rupture disc performance can be adversely affected through various installation, operation and maintenance practices. This article reviews some of these practices, real-life observed consequences, and corrective or preventative measures that can improve rupture disc performance. Part 1 of this feature report (Chemical Engineering, March 2009, pp. 42–44) discusses situations where combinations of rupture discs with relief valves should be considered. Liquid service Liquid-full systems create a number of processing challenges, many of which apply to rupture discs. Pressure spikes and water hammer generated by rapid opening or closing of valves somewhere in the process frequently do affect the rupture disc. The typical rupture disc begins to respond to pressure in excess of the burst pressure in less than 1 millisecond. This means that a short-duration pressure spike that is not detectable by normal process instrumentation can and will affect the rupture disc. Ways to avoid pressure spike problems include: avoiding the use of rupture discs on long, liquid-filled lines, eliminating fast-closing-and-opening process valves, and using pressure accumulators to absorb the unavoidable pressure spikes. Common indications that you have a water hammer or another pressure spike problem include the following: • The rupture disc appears to burst at a pressure lower than the marked burst pressure • The rupture disc is only partially open • Ruptures are observed during or immediately after some non-steady state condition in the process For optimum rupture-disc performance, pay attention to installation, operation and maintenance Part 2